PTHREADS These notes are from LLNL Pthreads Tutorial

The Pthreads API is defined in the ANSI/IEEE POSIX standard. The subroutines which comprise the Pthreads API can be informally grouped into three major classes: 1)Thread management: Functions that work directly on threads - creating, detaching, joining, etc. They include functions to set/query thread attributes (joinable, scheduling etc.) 2) Mutexes: Functions that deal with synchronization, Mutex functions provide for creating, destroying, locking and unlocking mutexes. They are also supplemented by mutex attribute functions that set or modify attributes associated with mutexes. 3) Condition variables: The third class of functions address communications between threads that share a mutex. They are based upon programmer specified conditions. This class includes functions to create, destroy, wait and signal based upon specified variable values. Functions to set/query condition variable attributes are also included. Naming conventions: All identifiers in the threads library begin with pthread_ PTHREADS API

Compiling Threaded Programs Compiler / PlatformCompiler CommandDescription INTEL Linux icc -pthread C icpc -pthread C++ GNU Linux, AIX gcc -pthread GNU C g++ -pthread GNU C++ Include file: pthread.h

THREAD CREATION int pthread_create(pthread_t *restrict thread, const pthread_attr_t *restrict attr, void *(*start_routine)(void*), void *restrict arg); pthread_create arguments: thread: An opaque, unique identifier for the new thread returned by the subroutine. attr: An opaque attribute object that may be used to set thread attributes. You can specify a thread attributes object, or NULL for the default values. start_routine: the C routine that the thread will execute once it is created. arg: A single argument that may be passed to start_routine. NULL may be used if no argument is to be passed.

Terminating Threads There are several ways in which a Pthread may be terminated: The thread returns from its starting routine (the main routine for the initial thread). The thread makes a call to the pthread_exit subroutine. The thread is canceled by another thread via the pthread_cancel routine,The entire process is terminated. void pthread_exit(void *value_ptr); Typically, the pthread_exit() routine is called after a thread has completed its work and is no longer required to exist. If main() finishes before the threads it has created, and exits with pthread_exit(), the other threads will continue to execute. Otherwise, they will be automatically terminated when main() finishes. The programmer may optionally specify a termination status, which is stored as a void pointer for any thread that may join the calling thread. Cleanup: the pthread_exit() routine does not close files; any files opened inside the thread will remain open after the thread is terminated.

#include #define NUM_THREADS 5 void *PrintHello(void *threadid) { int tid; tid = (int)threadid; printf("Hello World! It's me, thread #%d!\n", tid); pthread_exit(NULL); } int main (int argc, char *argv[]) { pthread_t threads[NUM_THREADS]; int rc, t; for(t=0; t<NUM_THREADS; t++){ printf("In main: creating thread %d\n", t); rc = pthread_create(&threads[t], NULL, PrintHello, (void *)t); if (rc){ printf("ERROR; return code from pthread_create() is %d\n", rc); exit(-1); } pthread_exit(NULL); } Example

int pthread_join(pthread_t thread, void **value_ptr); The pthread_join() subroutine blocks the calling thread until the specified threadid thread terminates. The programmer is able to obtain the target thread's termination return status if it was specified in the target thread's call to pthread_exit(). A joining thread can match one pthread_join() call. It is a logical error to attempt multiple joins on the same thread. JOINING

Creating and Destroying Mutexes Mutex variables must be declared with type pthread_mutex_t, and must be initialized before they can be used. There are two ways to initialize a mutex variable:  Statically, when it is declared. For example: pthread_mutex_t mymutex = PTHREAD_MUTEX_INITIALIZER;  Dynamically, with the pthread_mutex_init() routine. This method permits setting mutex object attributes, attr. The mutex is initially unlocked. Routines pthread_mutex_init (mutex,attr) pthread_mutex_destroy (mutex) pthread_mutexattr_init (attr) pthread_mutexattr_destroy (attr)

Locking and Unlocking Mutexes pthread_mutex_lock (mutex) pthread_mutex_trylock (mutex) pthread_mutex_unlock (mutex) Routines

Condition Variables Condition variables provide yet another way for threads to synchronize. While mutexes implement synchronization by controlling thread access to data, condition variables allow threads to synchronize based upon the actual value of data. Without condition variables, the programmer would need to have threads continually polling (possibly in a critical section), to check if the condition is met. This can be very resource consuming since the thread would be continuously busy in this activity. A condition variable is a way to achieve the same goal without polling. A condition variable is always used in conjunction with a mutex lock.

Condition Variables: Example of Usage Main Thread Declare and initialize global data/variables which require synchronization (such as "count") Declare and initialize a condition variable object Declare and initialize an associated mutex Create threads A and B to do work Thread A Do work up to the point where a certain condition must occur (such as "count" must reach a specified value) Lock associated mutex and check value of a global variable Call pthread_cond_wait() to perform a blocking wait for signal from Thread-B. Note that a call to pthread_cond_wait() automatically and atomically unlocks the associated mutex variable so that it can be used by Thread-B. When signalled, wake up. Mutex is automatically and atomically locked. Explicitly unlock mutex Continue Thread B Do work Lock associated mutex Change the value of the global variable that Thread-A is waiting upon. Check value of the global Thread-A wait variable. If it fulfills the desired condition, signal Thread-A. Unlock mutex. Continue Main Thread Join / Continue

Creating and Destroying Condition Variables Condition variables must be declared with type pthread_cond_t, and must be initialized before they can be used. There are two ways to initialize a condition variable:  Statically, when it is declared. For example: pthread_cond_t myconvar = PTHREAD_COND_INITIALIZER;  Dynamically, with the pthread_cond_init() routine. The ID of the created condition variable is returned to the calling thread through the condition parameter. This method permits setting condition variable object attributes, attr. Routines: pthread_cond_init (condition,attr) pthread_cond_destroy (condition) pthread_condattr_init (attr) pthread_condattr_destroy (attr)

Waiting and Signaling on Condition Variables pthread_cond_wait() blocks the calling thread until the specified condition is signalled. This routine should be called while mutex is locked, and it will automatically release the mutex while it waits. After signal is received and thread is awakened, mutex will be automatically locked for use by the thread. The programmer is then responsible for unlocking mutex when the thread is finished with it. The pthread_cond_signal() routine is used to signal (or wake up) another thread which is waiting on the condition variable. It should be called after mutex is locked, and must unlock mutex in order for pthread_cond_wait() routine to complete. The pthread_cond_broadcast() routine should be used instead of pthread_cond_signal() if more than one thread is in a blocking wait state. It is a logical error to call pthread_cond_signal() before calling pthread_cond_wait().

Proper locking and unlocking of the associated mutex variable is essential when using these routines. For example:  Failing to lock the mutex before calling pthread_cond_wait() may cause it NOT to block.  Failing to unlock the mutex after calling pthread_cond_signal() may not allow a matching pthread_cond_wait() routine to complete (it will remain blocked). Routines Waiting and Signaling on Condition Variables pthread_cond_wait (condition,mutex) pthread_cond_signal (condition) pthread_cond_broadcast (condition)

Example Code - Using Condition Variables This simple example code demonstrates the use of several Pthread condition variable routines. The main routine creates three threads. Two of the threads perform work and update a "count" variable. The third thread waits until the count variable reaches a specified value. #include #define NUM_THREADS 3 #define TCOUNT 10 #define COUNT_LIMIT 12 int count = 0; int thread_ids[3] = {0,1,2}; pthread_mutex_t count_mutex; pthread_cond_t count_threshold_cv;

int main (int argc, char *argv[]) { int i, rc; pthread_t threads[3]; pthread_attr_t attr; /* Initialize mutex and condition variable objects */ pthread_mutex_init(&count_mutex, NULL); pthread_cond_init (&count_threshold_cv, NULL); /* For portability, explicitly create threads in a joinable state */ pthread_attr_init(&attr); pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE); pthread_create(&threads[0], &attr, inc_count, (void *)&thread_ids[0]); pthread_create(&threads[1], &attr, inc_count, (void *)&thread_ids[1]); pthread_create(&threads[2], &attr, watch_count, (void *)&thread_ids[2]);

/* Wait for all threads to complete */ for (i=0; i<NUM_THREADS; i++) { pthread_join(threads[i], NULL); } printf ("Main(): Waited on %d threads. Done.\n", NUM_THREADS); /* Clean up and exit */ pthread_attr_destroy(&attr); pthread_mutex_destroy(&count_mutex); pthread_cond_destroy(&count_threshold_cv); pthread_exit(NULL); } /* of main /

void *inc_count(void *idp) { int j,i; double result=0.0; int *my_id = idp; for (i=0; i<TCOUNT; i++) { pthread_mutex_lock(&count_mutex); count++; /* Check the value of count and signal waiting thread when condition is reached. Note that this occurs while mutex is locked. */ if (count == COUNT_LIMIT) { pthread_cond_signal(&count_threshold_cv); printf("inc_count(): thread %d, count = %d Threshold reached.\n", *my_id, count); } printf("inc_count(): thread %d, count = %d, unlocking mutex\n",*my_id, count); pthread_mutex_unlock(&count_mutex); /* Do some work so threads can alternate on mutex lock */ for (j=0; j<1000; j++) result = result + (double)random(); } pthread_exit(NULL); }

void *watch_count(void *idp) { int *my_id = idp; printf("Starting watch_count(): thread %d\n", *my_id); /* Lock mutex and wait for signal. Note that the pthread_cond_wait routine will automatically and atomically unlock mutex while it waits. Also, note that if COUNT_LIMIT is reached before this routine is run by the waiting thread, the loop will be skipped to prevent pthread_cond_wait from never returning. */ pthread_mutex_lock(&count_mutex); if (count<COUNT_LIMIT) { pthread_cond_wait(&count_threshold_cv, &count_mutex); printf("watch_count(): thread %d Condition signal received.\n", *my_id); } pthread_mutex_unlock(&count_mutex); pthread_exit(NULL); }

inc_count(): thread 0, count = 1, unlocking mutex Starting watch_count(): thread 2 inc_count(): thread 1, count = 2, unlocking mutex inc_count(): thread 0, count = 3, unlocking mutex inc_count(): thread 1, count = 4, unlocking mutex inc_count(): thread 0, count = 5, unlocking mutex inc_count(): thread 0, count = 6, unlocking mutex inc_count(): thread 1, count = 7, unlocking mutex inc_count(): thread 0, count = 8, unlocking mutex inc_count(): thread 1, count = 9, unlocking mutex inc_count(): thread 0, count = 10, unlocking mutex inc_count(): thread 1, count = 11, unlocking mutex inc_count(): thread 0, count = 12 Threshold reached. inc_count(): thread 0, count = 12, unlocking mutex watch_count(): thread 2 Condition signal received. inc_count(): thread 1, count = 13, unlocking mutex inc_count(): thread 0, count = 14, unlocking mutex inc_count(): thread 1, count = 15, unlocking mutex inc_count(): thread 0, count = 16, unlocking mutex inc_count(): thread 1, count = 17, unlocking mutex inc_count(): thread 0, count = 18, unlocking mutex inc_count(): thread 1, count = 19, unlocking mutex inc_count(): thread 1, count = 20, unlocking mutex Main(): Waited on 3 threads. Done.